Bad weather? Blame Santa

If you’re fed up with floods in England, sick of snow in the US or mystified by mild temperatures in Scandinavia, blame it on Santa Claus. That’s the message coming from atmospheric scientist Jennifer Francis, whose “Santa’s revenge” hypothesis suggests that the weather weirdness that we’re currently seeing at middle latitudes could be linked to recent warming in the Arctic.

Francis’ theory begins with the polar jet stream, the high-altitude “river of air” that flows over parts of the northern hemisphere. This jet stream owes its existence to the temperature differential between the Arctic region and middle latitudes: because warm air expands, that temperature differential produces a “hill” of air with (for example) England at the top and Greenland at the bottom. The Earth’s rotation means that air doesn’t flow straight down this hill; instead, it curves around, producing the west–east flow seen in animations like the one in this video from the NASA Goddard Science Visualization Studio.

As the Arctic has warmed, however, the slope of the “hill” has decreased. The result, Francis argues, is that the jet stream is no longer behaving like a river rushing down a mountain. Instead, it’s become wavier and slower, like a river meandering through a delta. Her research also suggests that the jet stream is increasingly getting stuck in a particular pattern – such as the one that’s been sending storm after storm to the UK ever since mid-December 2013. Listen below for a more detailed explanation Francis gave to me and another UK-based journalist.

Santa's revenge

Atmospheric scientist Jennifer Francis explains how the UK's recent bad weather fits into her theory of Arctic warming and the jet stream

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Francis’ theory is considered controversial among atmospheric scientists, including some who have questioned the evidence in favour of a wavier jet stream. And, like much of the research presented here at the AAAS meeting, it isn’t especially new. But with a spate of weird weather creating headaches for people and governments across the northern hemisphere, it certainly feels newly relevant.